President Donald Trump’s former personal attorney, Michael Cohen, has participated over the last month in multiple interview sessions lasting for hours with investigators from the office of special counsel, Robert Mueller, sources tell ABC News.

The special counsel’s questioning of Cohen, one of the president’s closest associates over the past decade, has focused primarily on all aspects of Trump’s dealings with Russia — including financial and business dealings and the investigation into alleged collusion with Russia by the Trump campaign and its surrogates to influence the outcome of the 2016 presidential election, sources familiar with the matter tell ABC News.

Investigators were also interested in knowing, the sources say, whether Trump or any of his associates discussed the possibility of a pardon with Cohen.

Over the 16 months that Mueller has been investigating, the president has repeatedly bashed the investigation as a partisan witch hunt, insisting there has been no collusion and no obstruction of justice.

The interviews with Cohen took place in Washington, D.C., and New York City. They were also attended in part by prosecutors from the U.S. Attorney’s Office in the Southern District of New York.

Cohen’s participation in the meetings has been voluntary — without any guarantee of leniency from prosecutors, according to several people familiar with the situation.